pneumatic actuator sizing - regulated air supply pressure


Is it enough to size an pneumatic actuator based on a regulated air supply pressure, or i need to have minimum and maximum air supply pressure values? pressure is regulated by a PCV and there is a pressure gauge downstream of PCV reading the air pressure.  

need to replace valve and actuator and do not have other info with regards to air supply, except for what is mentioned above (taken from field)

please advise from experience.

Thank you

  • Hi John,
    Your question is a bit short of data however it would be considered good practice to know and consider both values. In short, the minmum air supply must be known in order to determine the lowest pressure available to power the actuator. The maximum pressure value should be known in order to ensure that if the actuator does become exposed to that value that it can withstand that maximum value. It would be strongly suggested that any actuator that receives a regulated pressure also be equipped with an appropriately sized relief valve. This relief valve should prevent the actuator from being exposed to a pressure which is greater that it's MOP. Of course the relief valve may not be needed if the actuator has a higher pressure rating (MOP) than the maximum air supply.
    Second - Remember that if an actuator has been sized to a minmum air supply pressure and then receives a higher pressure the torque or force output would likely increase. This could lead to damaging the valve or hardware due to ""overloading or exceeding the design parameters"" of those components.
    I suggest that you allow your qualified Emerson provider to select the replacement actuator and advise them of the minmum and maximum air supply available.
    Tom J.
  • Hi John,

    In addition to Tom's response, to which I mostly agree, a few remarks.

    The moment you include a regulator in a pneumatic system you will have to ask yourself:

    1) what will happen if if fails (open), and

    2) what will happen if a technician will try to 'solve' a problem by adjusting the correct set value (provided it was documented anywhere and readible from a reliable gauge)?

    Don't forget, installing and maintaining a regulator costs money as well.

    My personal opinion: The best regulator is no regulator.

    I can think of (only) five reasons why you would ever want to consider installing one:

    • Someone (?) decided to select an underrated pneumatic cylinder which needs protection against overpressure (and that will require a relief valve which will require periodic inspection as well)
    • The MAST of the valve is not capable of handling the output torque at the maximum pressure in the air supply system (no relief valve would be required since no parts will be flying around, only valve damage may happen)
    • The valve is a fail open butterfly valve (which means that you want a controlled air closing torque, otherwise you may damage the valve seat or the actuator spring torque may not be high enough to open the valve, no relief required)
    • Conventional methods to reduce the stroking time are not sufficient (adding a regulator with a setting just above the end of stroke pressure will reduce the pneumatic saturation and almost eliminate the dead time, no relief required)
    • The actuator is fed from a secure instrument air system (adding a regulator with a setting just above the end of stroke pressure will dramatically reduce the air consumption per stroke, no relief required).

    In case anyone can think of more valid reasons to install a regulator, please let me know.

    If not: The best regulator is no regulator.

    Good luck and regards, Frans

    Frans Martens

    ‘The Other Ways’

    Consultant Field Instrumentation and Systems

    The Netherlands